Vortex generators

[Sonex1517]

Bottom on my Sonex. The Waiex - that would be interesting!

[NWade]

Thanks for bravely posting your results, Robbie!

I've been down with a back injury the last few weeks. But once my plane is flying I hope to do some airflow tests similar to what Dick Johnson did for sailplanes in the 1970's through early 90's. I'm betting there's a laminar separation bubble happening on the top surface of the wing and the VGs are helping to "fix" it. But there may be a more-optimal position for them and something like sailplane zigzag tape may provide a good solution (depending on the thickness of the boundary layer at the separation point). Will certainly post my results once I get into flight test.

Take care,

--Noel

[GraemeSmith]

Interesting report Robbie. Seriously. I really think these things ARE worthy of examination.

MOST of the flight characteristics you report - I get without VG's. The "oil canning" on the stall - I call that the "burble". I haven't been able to observe any skins actually oil canning - so I'll go with it being the air "burbling". I call it my "audible stall warning". It's very handy when flying acro as it let's me know if I've pulled too hard and caused an accelerated stall on the wing.

I would be interested in what speed decay you are getting in 60 degree steep turns compared to before. I've got less hp than you - which is a factor - but I really struggle to keep up speed in steep-steep turns while holding altitude.

And are you painted or polished? Cause I can see polishing around them could be a real drag (pun intended)

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Perhaps we could both wool tuft our wings and video them - that would show the separation point and we could compare to see the difference. I'm thinking of doing that anyway to get a real sense of the stall and to try and calibrate my AOA better.

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And constructively / interested in observations - I would be very leery of putting VG's on the horizontal stab. It might give control authority longer - but if it prevents the stab from stalling then the plane will not nose over so easily when the main wing stalls - that means you might have to positively push to execute a stall recovery when previously the plane would have dropped its nose.

[Bryan Cotton]

And constructively / interested in observations - I would be very leery of putting VG's on the horizontal stab. It might give control authority longer - but if it prevents the stab from stalling then the plane will not nose over so easily when the main wing stalls - that means you might have to positively push to execute a stall recovery when previously the plane would have dropped its nose.

Do you ever want the tail to stall in a conventional (non-canard) airplane? I would think not. Stalled means no control for the tail.

Thanks Robbie for the written observations.

Noel, look forward to your zig zag tape experiment.

[GraemeSmith]

And constructively / interested in observations - I would be very leery of putting VG's on the horizontal stab. It might give control authority longer - but if it prevents the stab from stalling then the plane will not nose over so easily when the main wing stalls - that means you might have to positively push to execute a stall recovery when previously the plane would have dropped its nose.

Do you ever want the tail to stall in a conventional (non-canard) airplane? I would think not. Stalled means no control for the tail.

ABSOLUTLEY you do. It is an UPSIDE DOWN aerofoil - providing a DOWN force to keep the nose heavy aircraft level. (Hence as you load an aircraft into further aft CG - less down force with its induced drag is required and the aircraft flies faster or more efficiently - all other things remaining equal.)

In level flight in designed CG range - when the aircraft flies slowly and stalls - what SHOULD happen is that the horizontal stab stops providing enough down force to keep the nose up because it stalls or at least provides not enough "down lift". The tail rises and the nose drops and the plane accelerates out of a stall.

So anything that keeps the horizontal stab flying "better" and providing more efficient down force - compromises your ability to have the nose drop and fly out of a stall.

[Bryan Cotton]

So that would imply the stabilizer could stall before the wing. What happens then?

A classic stall removes lift from the wing, causing the nose to drop.

Edit: lots of stuff out there about deep stall, generally related to T tails or icing. Here is one article.
https://www.aviationsafetymagazine.com/ ... ne-stalls/

Having worked for two airframers in my career, I'm going to stick with the tail should never stall.

[tx_swordguy]

Robbie, Thanks for the report. I was very happy with the VGs on my avid and glad you are happy putting them on the sonex. I expected the changes in the stall for you and I expect a more solid turn to final with little to no bobble if you get close to the "stall" speed in the turn. I found my avid just mushed down in altitude but never truly stalled. I could be nose high with virtually no indicated (at least not accurate) airspeed and it felt like I had turning control and it was stable but I was dropping altitude. It was definitely a strange feeling but all you had to do was drop the nose or give power and you were in complete control. I did not expect the solidness you are describing in the steep turns at low air speed. That is a definate win
Mark

[Sonex1517]

I’m polished, and Bryan, since I used double sided tape I am trying to be careful not to make it turn black. Right now my leading edges and prop have dead bugs all over. A good thing as it means I’ve been flying. A bad thing because I have a lot of clean up to do and no time to do it.

It really was the solid feel in a steep turn that was the eye opener. These VG’s work. I was very skeptical going into this.

I’ll close in saying I had no intention of doing the horizontal stabilizer or vertical stabilizer. Then someone much wiser and more experienced than I explained why I should. And It was all about when the tail stalled vs the wing. I’m very satisfied with the results.

It may not interest everyone, and I’m sure some people question me for doing it. Oh well, it’s my plane.

This is *exactly* why I built.

[Area 51%]

The tail stalling prematurely on the Cessna Cardinal is why they introduced a slotted stabilizer in the later models.

Probably more cost effective to go with the generators.

[Bryan Cotton]

The tail stalling prematurely on the Cessna Cardinal is why they introduced a slotted stabilizer in the later models.

One disadvantage of a flying tail, like the Cardinal or Cherokee - it's a straight airfoil. When you have an elevator you are essentially getting a highly cambered airfoil when you have elevator deflection.